As a front end circuit (tuner section) for a television receiver, front end circuits by which the channel can be switched over a wide range have been proposed. For example, a front end circuit is available wherein an antenna input is divided into three reception bands and a power supply voltage is selectively supplied to high frequency circuits for the respective reception bands such that tuning is carried out with an object channel and then the tuned output is supplied to a mixer circuit.
Also a front end circuit of the so-called low IF conversion type wherein the number of parts is reduced by forming the front end circuit as an IC and the intermediate frequency is set considerably lower than the reception frequency and another front end circuit of the so-called direct conversion type wherein a local oscillation frequency is set equal to the reception frequency to make the intermediate frequency zero are available. With the front end circuits of the low IF conversion type and the direct conversion type, since they become worse in terms of the image disturbance, intermediate frequency signals on the I axis and the Q axis are formed and the image disturbance signal is canceled by a phase process.
It is to be noted that, as prior art documents, for example, U.S. Pat. No. 4,851,796, U.S. Patent No. 20060189290, Japanese Patent No. 23615099 and Japanese Translation of PCT Application No. 2004-531967 are available.
However, according to the type wherein an antenna input is divided into three reception bands to carry out tuning with an object channel as described above, a very great number of parts are required, and besides, tracking adjustment is carried out by changing the shape of an air-cored coil of a tuning circuit. Therefore, the front end circuit of the type described encounters with a significant trouble in reduction of the size and the cost.
In this connection, if the front end circuit of the type described is formed as an IC, the number of parts can be reduced. However, in a system which adopts a wide-band high-frequency amplifier, a great dynamic range becomes required in order to satisfy the demand for a multi-channel disturbance characteristic, and it is very difficult to achieve both of reduction of the voltage and reduction of power consumption and characteristics of the gain and the noise factor NF.
This invention contemplates solution to such problems as described above.